If confirmed by further research, this newly discovered effect - which is not seen in current climate prediction models - could significantly reduce estimates of future climate warming. Scientists from NASA's Goddard Space Flight Center in Greenbelt, Md., and the Massachusetts Institute of Technology present their findings in the March 2001 issue of the Bulletin of the American Meteorological Society.

"High clouds over the western tropical Pacific Ocean seem to systematically decrease when sea surface temperatures are higher," says Arthur Y. Hou of Goddard's Data Assimilation Office. Hou and co-authors Ming-Dah Chou of Goddard's Climate and Radiation Branch and Richard S. Lindzen of MIT analyzed satellite observations over the vast ocean region, which stretches from Australia and Japan nearly to the Hawaiian Islands.

The researchers compare this inverse relationship to the eye's iris, which opens and closes to counter changes in light intensity. The "adaptive infrared iris" of cirrus clouds opens and closes to permit the release of infrared energy, thus resisting warmer tropical sea surface temperatures, which occur naturally and are predicted to increase as the result of climate warming.

The study compares detailed daily observations of cloud cover from Japan's GMS-5 Geostationary Meteorological Satellite with sea surface temperature data from the U. S. National Weather Service's National Centers for Environmental Prediction over a 20-month period (January 1998 to August 1999).

The researchers found that cumulus cloud towers produced less cirrus clouds when they moved over warmer ocean regions. For each degree Celsius rise in ocean surface temperature, the ratio of cirrus cloud area to cumulus cloud area over the ocean dropped 17-27 percent. The observed range of surface temperatures beneath the clouds varied by 6.3 degrees Fahrenheit (3.5 degees C).

The authors propose that higher ocean surface temperatures directly cause the decline in cirrus clouds by changing the dynamics of cloud formation and rainfall.

Cirrus clouds - high-altitude clouds of ice crystals - typically form as a byproduct of the life cycle of cumulus towers created by rising updrafts of heated, moist air.

As these cumulus convective clouds grow taller, cloud water droplets collide and combine into raindrops and fall out of the cloud or continue to rise until they freeze into ice crystals and form cirrus clouds.

"With warmer sea surface temperatures beneath the cloud, the coalescence process that produces precipitation becomes more efficient," explains Lindzen. "More of the cloud droplets form raindrops and fewer are left in the cloud to form ice crystals. As a result, the area of cirrus cloud is reduced."

Clouds play a critical and complicated role in regulating the temperature of the Earth. Thick, bright, watery clouds like cumulus shield the atmosphere from incoming solar radiation by reflecting much of it back into space.

Thin, icy cirrus clouds are poor sunshields but very efficient insulators that trap energy rising from the Earth's warmed surface. A decrease in cirrus cloud area would have a cooling effect by allowing more heat energy, or infrared radiation, to leave the planet.

If this "iris effect" is found to be a general process active in tropical oceans around the world, the Earth may be much less sensitive to the warming effects of such influences as rising greenhouse gas concentrations in the atmosphere.

The researchers estimate that this effect could cut by two-thirds the projected increase in global temperatures initiated by a doubling of carbon dioxide in the atmosphere.

American Meteorological Society is the nation's leading professional society for scientists in the atmospheric, oceanic, and related sciences.

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